[#BloombergLP-bslmt-Once-callOnce-0f]
= xref:BloombergLP.adoc[BloombergLP]::xref:BloombergLP/bslmt.adoc[bslmt]::xref:BloombergLP/bslmt/Once.adoc[Once]::callOnce
:relfileprefix: ../../../
:mrdocs:

If no other thread has yet called `enter` or `callOnce`, then call the specified `function` and set this object to the state where pending and future calls to `enter` or `callOnce` will return `false` or do nothing, respectively. Otherwise, wait for the one‐time code to complete and return without calling `function` where `function` is a function or functor that can be called with no arguments. Note that one‐time code is considered not to have run if `function` terminates with an exception.

== Synopsis

Declared in `<bslmt_once.h>`

[source,cpp,subs="verbatim,replacements,macros,-callouts"]
----
template<class FUNC>
void
callOnce(FUNC& function);
----


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